JP7756165B2 - Non-oriented electrical steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a non-oriented electrical steel sheet and a manufacturing method thereof. More specifically, the present invention relates to a non-oriented electrical steel sheet with improved magnetic properties achieved by appropriately adjusting the contents of Sb, Sn, Cu, Cr, and Mg, and a manufacturing method thereof.

Non-oriented electrical steel sheets are primarily used in motors that convert electrical energy into mechanical energy, and the excellent magnetic properties of non-oriented electrical steel sheets are required to achieve high efficiency in this process. In particular, environmentally friendly technologies have been attracting attention in recent years, and increasing the efficiency of motors, which account for more than half of total electrical energy consumption, is considered extremely important. As a result, demand for non-oriented electrical steel sheets with excellent magnetic properties is also increasing.

The magnetic properties of non-oriented electrical steel sheet are primarily evaluated by iron loss and magnetic flux density. Iron loss refers to the energy loss that occurs at a specific magnetic flux density and frequency, while magnetic flux density refers to the degree of magnetization obtained under a specific magnetic field. Lower iron loss means that a motor with higher energy efficiency can be manufactured under the same conditions, and higher magnetic flux density means that motors can be made smaller or copper loss can be reduced, so it is important to create non-oriented electrical steel sheet with low iron loss and high magnetic flux density.

The characteristics of non-oriented electrical steel sheets that should be considered also vary depending on the operating conditions of the motor. For many motors, the most important criterion for evaluating the characteristics of non-oriented electrical steel sheets used in motors is W15 _/50 , which is the iron loss when a 1.5 T magnetic field is applied at a commercial frequency of 50 Hz. However, not all motors for various applications consider W15 _/50 iron loss to be the most important, and iron loss at other frequencies or applied magnetic fields may be evaluated depending on the main operating conditions. In particular, for non-oriented electrical steel sheets used in the drive motors of recent electric vehicles, magnetic properties at low magnetic fields of 1.0 T or less and high frequencies of 400 Hz or more are often important, so the characteristics of non-oriented electrical steel sheets are evaluated using iron loss such as W10 _/400 .

A commonly used method for improving the magnetic properties of non-oriented electrical steel sheet is to add alloying elements such as Si. Adding these alloying elements can increase the steel's resistivity, and the higher the resistivity, the lower the eddy current loss and the lower the overall iron loss. However, the more Si added, the lower the magnetic flux density and the greater the brittleness. Adding more than a certain amount makes cold rolling impossible, making commercial production impossible. While thinner electrical steel sheet is particularly effective in reducing iron loss, the reduced rollability due to brittleness can be a fatal problem. Adding elements such as Al and Mn to further increase the steel's resistivity allows the production of the highest quality non-oriented electrical steel sheet with excellent magnetic properties.

For non-oriented electrical steel sheets used in electric vehicle drive motors, high-frequency iron loss above 400 Hz is important. As the frequency increases, the proportion of eddy current loss in iron loss increases, making it advantageous to increase resistivity and reduce thickness. However, as the steel sheet becomes thinner, the cold reduction rate increases, which can lead to the development of {111} //ND texture and poor magnetic properties. If the thickness of the hot-rolled sheet is reduced and the cold reduction rate is decreased to address this issue, the shape of the steel sheet cannot be adequately controlled during the cold rolling process, increasing thickness deviation in the width direction and resulting in dimensional defects in the motor core. Furthermore, as the steel sheet becomes thinner, the coil length increases, which increases the operating time of the continuous annealing process and creating the problem of reduced annealing productivity.

In order to solve these problems, attempts have been made to thoroughly remove impurities during the steelmaking process to produce extremely clean steel, or to add specific elements to reduce inclusions and precipitates in the steel and improve magnetic properties, but these methods are limited in their practical application due to limitations in commercial production conditions. Additionally, methods have been proposed to improve texture by controlling the annealing temperature and atmosphere, and the deformation rate of the steel sheet during rolling, but the technologies in practical use are extremely limited due to factors such as increased manufacturing costs, reduced productivity, and insufficient results.

The object of the present invention is to provide a non-oriented electrical steel sheet and a manufacturing method thereof. Specifically, the present invention provides a non-oriented electrical steel sheet and a manufacturing method thereof in which the magnetic properties are improved by appropriately adjusting the contents of Sb, Sn, Cu, Cr, and Mg.

The non-oriented electrical steel sheet of the present invention is characterized by containing, by weight, Si: 3.0-4.0%, Al: 0.3-1.5%, Mn: 0.1-0.6%, one or more of Sn and Sb: 0.006-0.1%, C: 0.0015-0.0040%, Cr: 0.01-0.03%, Cu: 0.003-0.008%, and Mg: 0.0005-0.0025%, with the balance consisting of Fe and unavoidable impurities.

The non-oriented electrical steel sheet of the present invention can satisfy the following formula 1.
[Formula 1]
0.66≦([Sn]+[Sb])/([Cr]+[Cu]+[Mg])≦2
(In formula 1, [Sn], [Sb], [Cr], [Cu], and [Mg] represent the contents (wt%) of Sn, Sb, Cr, Cu, and Mg, respectively.)

The non-oriented electrical steel sheet of the present invention may further contain 0.0003 to 0.0030 wt.% each of one or more of N, S, Ti, Nb, and V.

The non-oriented electrical steel sheet of the present invention may further contain one or more of P: 0.005 to 0.05 wt%, Mo: 0.001 to 0.01 wt%, and Ni: 0.005 to 0.04 wt%.

The non-oriented electrical steel sheet of the present invention may have an average grain size of 55 to 75 μm.

The non-oriented electrical steel sheet of the present invention has an oxide layer extending from the surface toward the interior of the steel sheet, and the thickness of the oxide layer can be 10 to 50 nm.

The oxide layer may contain 1.0 to 30 wt. % Al and 0.5 to 10.0 wt. % Si.

The weight ratio of Al content to Si content in the oxide layer can be 5 to 20.

The distribution density of AlN precipitates having a diameter of 10 to 500 nm at a depth of 2 μm or less from the surface toward the interior of the steel plate may be 3 precipitates/mm ² or less.

The thickness of the steel plate can be 0.10 to 0.35 mm.

The method for producing non-oriented electrical steel sheet of the present invention is characterized by including the steps of: hot rolling a slab that contains, by weight, 3.0-4.0% Si, 0.3-1.5% Al, 0.1-0.6% Mn, 0.006-0.1% one or more of Sn and Sb, 0.0015-0.0040% C, 0.01-0.03% Cr, 0.003-0.008% Cu, and 0.0005-0.0025% Mg, with the balance being Fe and unavoidable impurities, and satisfying the following formula 1 to produce a hot-rolled sheet; cold-rolling the hot-rolled sheet to produce a cold-rolled sheet; and final annealing the cold-rolled sheet.

[Formula 1]
0.66≦([Sn]+[Sb])/([Cr]+[Cu]+[Mg])≦2
(In formula 1, [Sn], [Sb], [Cr], [Cu], and [Mg] represent the contents (wt%) of Sn, Sb, Cr, Cu, and Mg, respectively.)

The method may further include a step of heating the slab to 1200°C or less before the step of producing the hot-rolled sheet.

The finishing rolling temperature during the hot-rolled sheet production stage can be 800°C or higher.

After the step of producing the hot-rolled sheet, the method may further include a step of annealing the hot-rolled sheet at 850 to 1150°C.

The final annealing step can involve annealing the cold-rolled sheet at a soaking temperature of 900°C or higher for 15 seconds or more.

In the final annealing step, the cold-rolled sheet may be annealed in an atmosphere containing 40% by volume or less of hydrogen (H ₂ ) and 60% by volume or more of nitrogen, with a dew point of 0 to -40°C.

One embodiment of the present invention provides a non-oriented electrical steel sheet with excellent high-frequency iron loss, which can contribute to improving the performance of drive motors for environmentally friendly automobiles that use the highest quality non-oriented electrical steel sheet.

1 is a schematic diagram of a cross section of a non-oriented electrical steel sheet according to an embodiment of the present invention.

Terms such as first, second, and third are used to describe various parts, components, regions, layers, and/or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Therefore, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. As used in the specification, the term "comprises" refers to the inclusion of certain properties, regions, integers, steps, operations, elements, and/or components, and does not exclude the presence or inclusion of other properties, regions, integers, steps, operations, elements, and/or components.

When a part is referred to as being "on" or "above" another part, this may be directly on or above the other part, with other parts intervening. In contrast, when a part is referred to as being "directly on" another part, there are no other parts intervening.

Unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight.

In one embodiment of the present invention, the inclusion of an additional element means that an additional amount of the additional element is included in place of the remaining iron (Fe).

Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries are further analyzed to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not to be construed as having an ideal or overly formal meaning unless otherwise defined.

The following describes in detail an embodiment of the present invention so that a person skilled in the art can easily implement the present invention. However, the present invention can be embodied in various different forms and is not limited to the embodiments described herein.

A non-oriented electrical steel sheet according to one embodiment of the present invention contains, by weight, Si: 3.0-4.0%, Al: 0.3-1.5%, Mn: 0.1-0.6%, one or more of Sn and Sb: 0.006-0.1%, C: 0.0015-0.0040%, Cr: 0.01-0.03%, Cu: 0.003-0.008%, Mg: 0.0005-0.0025%, and the balance consisting of Fe and unavoidable impurities.

Below, we will explain the reasons for limiting the composition of non-oriented electrical steel sheets.

Si: 3.0 to 4.0% by weight
Silicon (Si) increases the resistivity of the material and reduces iron loss. If too little Si is added, the iron loss improvement effect may be insufficient. If too much Si is added, the brittleness of the material increases, rolling productivity drops sharply, and an oxide layer and oxides that are harmful to magnetic properties may form in the surface layer. Therefore, Si may be included in an amount of 3.0 to 4.0 wt. %. More specifically, it may be included in an amount of 3.1 to 3.8 wt. %.

Al: 0.3-1.5% by weight
Aluminum (Al) increases the resistivity of the material and reduces iron loss. If too little Al is added, fine nitrides are formed or the oxide layer on the surface is not dense enough, making it difficult to achieve magnetic improvement. If too much Al is added, excessive nitrides are formed, degrading magnetic properties and causing problems in all processes, including steelmaking and continuous casting, and significantly reducing productivity. Therefore, the Al content may be 0.30 to 1.50 wt. %. More specifically, the Al content may be 0.40 to 1.30 wt. %.

Mn: 0.1 to 0.6% by weight
Manganese (Mn) increases the resistivity of the material, improving core loss, and plays a role in forming sulfides. If too little Mn is added, fine sulfides are formed, causing magnetic deterioration. If too much Mn is added, fine MnS is excessively precipitated, promoting the formation of a {111} texture that is unfavorable to magnetic properties and resulting in a rapid decrease in magnetic flux density. Therefore, Mn can be included in an amount of 0.1 to 0.6 wt %. More specifically, Mn can be included in an amount of 0.2 to 0.5 wt %.

One or more of Sn and Sb: 0.006 to 0.100 wt%
Tin (Sn) and antimony (Sb) segregate at the surface and grain boundaries of a steel sheet to suppress surface oxidation during annealing, prevent element diffusion through grain boundaries, and prevent recrystallization in the {111}//ND orientation, thereby improving texture. If Sn and Sb are added in excessively small amounts, the above-mentioned effects are insufficient. If Sn and Sb are added in excessive amounts, the amount of grain boundary segregation increases, reducing toughness and productivity, which may be lower than the improvement in magnetic properties. Therefore, at least one of Sn and Sb may be included in an amount of 0.006 to 0.100 wt. More specifically, at least one of Sn and Sb may be included in an amount of 0.010 to 0.070 wt. The term "at least one of Sn and Sb" refers to the content of either Sn or Sb alone when Sn or Sb is included alone, and refers to the combined amount of Sn and Sb when both Sn and Sb are included.

C: 0.0015 to 0.0040% by weight
Carbon (C) causes magnetic aging and combines with other impurity elements to form carbides, which degrades magnetic properties, so the lower the C content, the better. However, in one embodiment of the present invention, Cr, Cu, and Mg are contained in appropriate amounts, and even if C is contained in a certain amount or more, it does not affect magnetic properties. Therefore, it can contain 0.0015 wt% or more. Specifically, it can contain 0.0015 to 0.0040 wt% C. Even more specifically, it can contain 0.0020 to 0.0035 wt% C.

Cr: 0.0100 to 0.0300% by weight
Chromium (Cr) does not tend to form fine precipitates, but it can prevent the formation of an Al-based oxide layer on the surface and form Cr-based carbides, which can deteriorate magnetic properties. If too little Cr is added, the Al-based oxide layer may become too thick or round-shaped oxides or nitrides may form on the surface, which can deteriorate magnetic properties. If too much Cr is added, it may be difficult to form a dense oxide layer, which can deteriorate magnetic properties. Therefore, the Cr content may be 0.0100 to 0.0300 wt. %. More specifically, the Cr content may be 0.0120 to 0.0275 wt. %.

Cu: 0.0030 to 0.0080% by weight
Copper (Cu) is an element that can form sulfides at high temperatures, and when added in large amounts, it also affects the composition of the surface oxide layer. Adding an appropriate amount has the effect of coarsening fine CuS or MnCuS precipitates, thereby improving magnetic properties. Therefore, Cu can be included in an amount of 0.0030 to 0.0080 wt. %. More specifically, Cu can be included in an amount of 0.0040 to 0.0077 wt. %.

Mg: 0.0005-0.0025% by weight
Magnesium (Mg) is an element that mainly combines with S to form sulfides, which can affect the surface oxide layer of the base iron. Therefore, Mg can be contained in an amount of 0.0005 to 0.0025 wt %. More specifically, Mg can be contained in an amount of 0.0008 to 0.0020 wt %.

The non-oriented electrical steel sheet according to an embodiment of the present invention satisfies the following formula 1.
[Formula 1]
0.66≦([Sn]+[Sb])/([Cr]+[Cu]+[Mg])≦2.00

More specifically, the value of Equation 1 may be 0.68 to 1.95.

The non-oriented electrical steel sheet according to one embodiment of the present invention may further contain 0.0003 to 0.0030 wt.% each of one or more of N, S, Ti, Nb, and V.

N: 0.0003 to 0.0030% by weight
Nitrogen (N) not only forms fine AlN precipitates inside the base material, but also combines with other impurities to form fine precipitates that inhibit grain growth and worsen iron loss, so the lower the N content, the better, and it can be contained at 0.0003 to 0.0030 wt %, and more preferably, it is controlled at 0.0005 to 0.0025 wt %.

S: 0.0003 to 0.0030% by weight
Sulfur (S) forms fine precipitates such as MnS, CuS, and (Mn, Cu)S, which deteriorate magnetic properties and hot workability, so it is best to keep the content low. Therefore, if S is further included, it can be contained in an amount of 0.0003 to 0.0030 wt. %. More specifically, it can be contained in an amount of 0.0005 to 0.0025 wt. %.

Ti: 0.0003 to 0.0030% by weight
Titanium (Ti) has a strong tendency to form precipitates in steel, forming fine carbides, nitrides, or sulfides inside the base material, which inhibit grain growth and deteriorate core loss. Therefore, the Ti content must be controlled to 0.004 % or less, preferably 0.002% or less.

Nb: 0.0003 to 0.0030% by weight
Niobium (Nb) forms fine carbides or nitrides inside the matrix, inhibiting grain growth and domain wall motion, thereby deteriorating core loss. Therefore, the Nb content must be controlled to 0.004 % or less, and more preferably 0.002% or less.

V: 0.0003 to 0.0030% by weight
Vanadium (V) forms fine carbides or nitrides inside the matrix, inhibiting grain growth and domain wall motion, thereby deteriorating core loss. Therefore, the V content must be controlled to 0.004 % or less, and more preferably 0.002% or less.

The non-oriented electrical steel sheet according to one embodiment of the present invention may further contain one or more of P: 0.005 to 0.05 wt %, Mo: 0.001 to 0.01 wt %, and Ni: 0.005 to 0.04 wt %.

P: 0.005-0.050% by weight
Phosphorus (P) segregates at the surface and grain boundaries of the steel sheet to suppress surface oxidation during annealing, prevent element diffusion through grain boundaries, and prevent recrystallization in the {111}//ND orientation, improving the texture. If too little P is added, these effects are insufficient. If too much P is added, hot working characteristics may deteriorate, resulting in lower productivity compared to magnetic improvement. Therefore, if P is further included, it may be included in an amount of 0.005 to 0.050 wt. More specifically, P may be included in an amount of 0.007 to 0.045 wt.

Mo: 0.001 to 0.01% by weight
Molybdenum (Mo) segregates at the surface and grain boundaries to improve texture. If too little Mo is added, the {111} texture develops, which can lead to poor magnetic properties. If too much Mo is added, it can suppress the segregation of Sn and P, reducing the texture improvement effect. Therefore, if Mo is further added, it can be included in an amount of 0.001 to 0.01 wt%.

Ni: 0.005-0.04% by weight
Nickel (Ni) increases the ductility of steel and promotes the segregation of Sn and P. If too much Ni is added, the magnetic flux density may drop sharply. Therefore, when Ni is further added, it may be included in an amount of 0.005 to 0.04 wt%.

The balance consists of Fe and unavoidable impurities. unavoidable impurities are impurities that are mixed in during the steelmaking stage and the manufacturing process of grain-oriented electrical steel sheets, and as this is widely known in the relevant field, a detailed description of this will be omitted. This does not preclude the addition of elements other than the alloy components described above in one embodiment of the present invention, and various elements may be included within a range that does not impair the technical concept of the present invention. When additional elements are further included, they are included in place of the balance of Fe.

Inevitable impurities include, for example, B and Zr, and must be controlled to B: 0.002 wt% or less, and Zr: 0.005 wt% or less.

Figure 1 shows a cross section of a non-oriented electrical steel sheet according to one embodiment of the present invention. As shown in Figure 1, an oxide layer 20 exists from the surface toward the interior of the electrical steel sheet 100. The electrical steel sheet 100 excluding the oxide layer 20 is classified as the base material 10 of the electrical steel sheet.

The electrical steel sheet 100 is exposed to oxygen during the manufacturing process, and oxygen in the atmosphere penetrates into the steel sheet, resulting in an oxygen concentration gradient from the surface toward the interior.

The oxide layer 20 and the base material 10 are divided into an oxide layer 20 with an oxygen content of 40% or more by weight and a base material 10 with an oxygen content of less than 40% by weight. The thickness of the oxide layer 20 thus divided may be 10 to 50 nm. By forming an oxide layer 20 of such an appropriate thickness, the diffusion of nitrogen in the atmosphere during annealing into the base material is suppressed, thereby suppressing the formation of fine nitrides, thereby improving magnetic properties. The thickness of the oxide layer 20 across the entire surface of the steel sheet may vary, and in one embodiment of the present invention, the thickness of the oxide layer 20 refers to the average thickness within the steel sheet.

In addition to oxygen present due to oxygen penetration during the manufacturing process, this oxide layer 20 contains a large amount of Al that has diffused into and concentrated in the base material 10. On the other hand, the increase in Al and O can result in a relative decrease in the Si content.

Specifically, the oxide layer 20 can contain 1.0 to 30 wt% Al and 0.5 to 10.0 wt% Si. More specifically, the oxide layer 20 can contain 40 to 70 wt% O, 1 to 30 wt% Al, and 0.5 to 10.0 wt% Si, with the remainder being Fe and unavoidable impurities. The formation of an oxide layer with concentrated Al in this way can suppress the formation of rounded oxides and fine nitrides within the base material, improving magnetic properties. As with O, a concentration gradient can exist for Al, with the content increasing from the base material toward the surface, and the aforementioned range refers to the average content in the oxide layer 20.

The weight ratio of the Al content to the Si content in the oxide layer 20 can be 5 to 20. Increasing the amount of Al in the oxide layer 20 in this way forms a dense oxide layer, suppressing the formation of fine precipitates below the surface that can occur during the final annealing process, thereby achieving excellent magnetic properties. More specifically, the weight ratio of the Al content to the Si content in the oxide layer 20 can be 7.0 to 17.0.

The non-oriented electrical steel sheet according to an embodiment of the present invention may have an average grain size of 55 to 75 μm. The magnetic properties of the non-oriented electrical steel sheet are superior within the above range. The grain size is calculated by (measurement area/number of grains) ^0.5 . The grain size may be measured on the basis of a plane parallel to the rolled surface (ND plane) and may be measured within the base material 10. Specifically, the average grain size may be 60 to 70 μm.

In one embodiment of the present invention, the density of AlN precipitates in the surface region can be reduced by appropriately controlling the alloying elements. Specifically, the distribution density of AlN precipitates with diameters of 10 to 500 nm at a depth of 2 μm or less from the surface of the steel sheet toward the interior can be 3 precipitates/ ^mm² or less. By reducing the distribution density of AlN inclusions in this way, fine precipitates that hinder domain wall motion can be suppressed, contributing to improved magnetic properties. More specifically, the distribution density of AlN precipitates can be 0.5 to 2.5 precipitates/ ^mm² . The diameter of the AlN can be measured based on a plane parallel to the rolled surface (ND plane). The diameter of the AlN can be calculated by assuming a circle with the same area as the AlN.

The thickness of the steel plate can be 0.10 to 0.35 mm.

As described above, one embodiment of the present invention provides an optimal alloy composition, which improves precipitate properties and enhances magnetic properties. Specifically, the iron loss (W10 _/400 ) of the non-oriented electrical steel sheet is 12.5 W/kg or less, and the magnetic flux density ( _B50 ) is 1.650 T or more. The iron loss (W10/400) is the iron loss when a magnetic flux density of 1.0 T is induced at a frequency of 400 Hz. The magnetic flux density ( _B50 ) is the magnetic flux density induced in a magnetic field of 5000 A/m. More specifically, the iron loss ( _{W10 /400} ) of the non-oriented electrical steel sheet may be 11.6 W/kg or less, and the magnetic flux density ( _B50 ) may be 1.660 T or more.

A method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention includes the steps of hot-rolling a slab to produce a hot-rolled sheet, cold-rolling the hot-rolled sheet to produce a cold-rolled sheet, and final annealing the cold-rolled sheet.

First, the slab is hot rolled.

The alloying elements of the slabs were explained above in the section on alloying elements of non-oriented electrical steel sheets, so a duplicate explanation will be omitted. Since the alloying elements do not substantially change during the manufacturing process of non-oriented electrical steel sheets, the alloying elements of non-oriented electrical steel sheets and slabs are substantially identical.

Specifically, the slab contains, by weight , 3.0 to 4.0% Si, 0.3 to 1.5% Al, 0.1 to 0.6% Mn, 0.006 to 0.1% of one or more of Sn and Sb, 0.0015 to 0.0040% C, 0.01 to 0.03% Cr, 0.003 to 0.008% Cu, and 0.0005 to 0.0025% Mg, with the remainder being Fe and unavoidable impurities, and satisfies the above formula 1.

Other additional elements have been explained in the section on alloying elements for non-oriented electrical steel sheets, so duplicate explanations will be omitted.

The slab is heated before hot rolling. There are no restrictions on the heating temperature of the slab, but it should be heated to 1200°C or less. If the heating temperature of the slab is excessively high, precipitates such as AlN and MnS present in the slab will be redissolved and then finely precipitated during hot rolling and annealing, which can inhibit grain growth and reduce magnetic properties.

The slab is then hot-rolled to produce a hot-rolled sheet. The thickness of the hot-rolled sheet is 2 to 2.3 mm. The finish rolling temperature in the hot-rolled sheet production stage can be 800°C or higher. Specifically, it can be 800 to 1000°C. The hot-rolled sheet can be coiled at a temperature of 700°C or lower.

After the step of producing the hot-rolled sheet, the method may further include a step of annealing the hot-rolled sheet. The annealing temperature for the hot-rolled sheet may be 850 to 1150°C. If the annealing temperature for the hot-rolled sheet is too low, the structure may not grow or may grow too fine, making it difficult to obtain a texture favorable for magnetic properties during annealing after cold rolling. If the annealing temperature is too high, the magnetic crystal grains may grow excessively, resulting in excessive surface defects in the sheet. Annealing of the hot-rolled sheet may be performed as needed to increase the orientation favorable for magnetic properties, or may be omitted. The annealed hot-rolled sheet may be pickled.

The hot-rolled sheet is then cold-rolled to produce a cold-rolled sheet. Cold rolling is performed to a final thickness of 0.1 mm to 0.35 mm. The reduction ratio during the cold rolling step can be adjusted to 85% or more. More specifically, the reduction ratio can be 85 to 95%. If the reduction ratio is too low, thickness variations across the width of the steel sheet may occur.

Next, the cold-rolled sheet undergoes final annealing. The cold-rolled sheet is annealed by maintaining a soaking temperature of 900°C or higher for at least 15 seconds. Since the iron loss of non-oriented electrical steel sheet is closely related to the size of the crystal grains, it is annealed at an appropriate temperature and time. More specifically, it is annealed for 30 to 150 seconds at a soaking temperature of 950 to 1100°C.

In the final annealing step, the cold-rolled sheet is annealed in an atmosphere containing 40% or less by volume of hydrogen (H ₂ ) and 60% or more by volume of nitrogen, with a dew point of 0 to -40°C. Specifically, the annealing is performed in an atmosphere containing 5 to 40% by volume of hydrogen and 60 to 95% by volume of nitrogen. During the final annealing process, the average grain size becomes 55 to 75 μm, and all (i.e., 99% or more) of the worked structure formed in the previous cold rolling step is recrystallized.

After final annealing, an insulating coating is formed. This insulating coating can be made of organic, inorganic, or organic/inorganic composite coatings, and can also be made of other insulating coating agents.

The present invention will be described in more detail below using examples. However, these examples are merely for the purpose of illustrating the present invention and are not intended to limit the scope of the present invention.

Slabs were produced using the components shown in Tables 1 and 2, with the remainder consisting of Fe and unavoidable impurities. They were heated to 1150°C and hot rolled at a finishing temperature of 830°C to produce hot-rolled sheets with a thickness of 2.3 mm. The hot-rolled sheets were then annealed at 1030°C for 100 seconds, cold-rolled to a thickness of 0.27 mm, and recrystallized at 950°C for 88 seconds.

Table 3 shows the thickness of the oxide layer, the Al and Si contents in the oxide layer, the distribution density of AlN in the surface layer, W _10/400 iron loss, and B ₅₀ magnetic flux density for each test piece.

The thickness of the oxide layer was measured by processing the test piece with an FIB to create a smooth cross section, then photographing this with a high-magnification TEM and measuring the thickness of the oxide layer at more than 10 points on the surface of the base material.

The thickness of the oxide layer was measured at 10 or more points on the surface of the base material by processing the test piece with an FIB to produce a smooth cross section, photographing this at a high magnification using a TEM, and the average value was calculated.

For magnetic properties such as magnetic flux density and iron loss, five test pieces measuring 60 mm wide x 60 mm long were cut from each test piece, and measurements were taken in the rolling direction and the direction perpendicular to the rolling direction using a single sheet tester, and the average values were calculated. Here, W10 _/400 is the iron loss when a magnetic flux density of 1.0 T is induced at a frequency of 400 Hz, and _B50 is the magnetic flux density induced in a magnetic field of 5000 A/m.

As shown in Tables 1 to 3, in the cases of A4, B4, C3, C4, D3, and D4, in which the alloy components are appropriately controlled, an oxide layer is appropriately formed, and less AlN is formed, resulting in excellent magnetic properties.

On the other hand, A1 contains too little Cr, which prevents the oxide layer from forming properly, resulting in the formation of a large amount of AlN, resulting in poor magnetic properties.

It can be seen that A2 contains too little Mg, preventing the oxide layer from forming properly and resulting in the formation of a large amount of AlN, resulting in poor magnetic properties.

It can be seen that in A3, the value of Equation 1 is excessively large, so the oxide layer does not form properly, and a large amount of AlN is formed, resulting in poor magnetic properties.

B1 contains large amounts of Sn and Sb, and the value of Equation 1 is excessively large, so the oxide layer cannot be formed properly, and a large amount of AlN is formed, resulting in poor magnetic properties.

It can be seen that B2 contains a large amount of Mg, which prevents the oxide layer from forming properly, resulting in the formation of a large amount of AlN and poor magnetic properties.

In B3, the value of Equation 1 is too small, so the oxide layer does not form properly, and a large amount of AlN is formed, resulting in poor magnetic properties.

It can be seen that C1 contains a large amount of Cu, which prevents the oxide layer from forming properly, resulting in the formation of a large amount of AlN and poor magnetic properties.

C2 contains small amounts of Sn and Sb, and the value of Equation 1 is too small, so the oxide layer cannot be formed properly, and a large amount of AlN is formed, resulting in poor magnetic properties.

It can be seen that D1 contains too little Cu, preventing the oxide layer from forming properly and resulting in the formation of a large amount of AlN, resulting in poor magnetic properties.

D2 contains an excessive amount of Cr, which prevents the oxide layer from forming properly, resulting in the formation of a large amount of AlN and poor magnetic properties.

It can be seen that D5 contains too little Al, which prevents the oxide layer from forming properly and results in poor magnetic properties.

The present invention is not limited to the examples and can be manufactured in a variety of different forms. Those skilled in the art will understand that the present invention can be embodied in other specific forms without changing the technical concept or essential characteristics of the present invention. Therefore, the above example should be understood to be illustrative in all respects and not limiting.

100 Non-oriented electrical steel sheet 10 Base material 20 Oxide layer

Claims (14)

The alloy contains, by weight, 3.0 to 4.0% Si, 0.3 to 1.5% Al, 0.1 to 0.6% Mn, 0.006 to 0.1% total of one or more of Sn and Sb, 0.0015 to 0.0040% C, 0.01 to 0.03% Cr, 0.003 to 0.008% Cu, and 0.0005 to 0.0025% Mg, with the balance being Fe and unavoidable impurities, and satisfies the following formula 1:
A non-oriented electrical steel sheet characterized in that the distribution density of AlN precipitates having a diameter of 10 to 500 nm at a depth of 2 μm or less from the surface of the steel sheet toward the interior is 3 precipitates/mm2 or less ^.
[Formula 1]
0.66≦([Sn]+[Sb])/([Cr]+[Cu]+[Mg])≦2.00
(In formula 1, [Sn], [Sb], [Cr], [Cu], and [Mg] represent the contents (wt%) of Sn, Sb, Cr, Cu, and Mg, respectively.) The non-oriented electrical steel sheet according to claim 1, further comprising 0.0003 to 0.0030 wt.% of one or more of N, S, Ti, Nb, and V. The non-oriented electrical steel sheet according to claim 1 or 2, further comprising one or more of P: 0.005-0.05 wt%, Mo: 0.001-0.01 wt%, and Ni: 0.005-0.04 wt%. A non-oriented electrical steel sheet according to any one of claims 1 to 3, characterized in that the average grain size is 55 to 75 μm. A non-oriented electrical steel sheet according to any one of claims 1 to 4, characterized in that an oxide layer is present from the surface of the steel sheet toward the interior, and the thickness of the oxide layer is 10 to 50 nm. The non-oriented electrical steel sheet according to claim 5, characterized in that the oxide layer contains 1.0 to 30% by weight of Al and 0.5 to 10.0% by weight of Si. The non-oriented electrical steel sheet according to claim 5 or 6, characterized in that the weight ratio of the Al content to the Si content in the oxide layer is 5 to 20. The non-oriented electrical steel sheet according to any one of claims 1 to 7 , characterized in that the thickness is 0.10 to 0.35 mm. a step of manufacturing a hot-rolled sheet by hot-rolling a slab containing, by weight %, 3.0 to 4.0% Si, 0.3 to 1.5% Al, 0.1 to 0.6% Mn, 0.006 to 0.1% total of one or more of Sn and Sb, 0.0015 to 0.0040% C, 0.01 to 0.03% Cr, 0.003 to 0.008% Cu, and 0.0005 to 0.0025% Mg, with the balance being Fe and unavoidable impurities, and satisfying the following formula 1:
cold-rolling the hot-rolled sheet to produce a cold-rolled sheet; and final- annealing the cold-rolled sheet,
A method for producing a non-oriented electrical steel sheet, characterized in that the distribution density of AlN precipitates having a diameter of 10 to 500 nm at a depth of 2 μm or less from the surface of the steel sheet toward the interior is 3 precipitates/mm2 ^or less .
[Formula 1]
0.66≦([Sn]+[Sb])/([Cr]+[Cu]+[Mg])≦2.00
(In formula 1, [Sn], [Sb], [Cr], [Cu] and [Mg] represent the contents (wt%) of Sn, Sb, Cr, Cu and Mg in the slab, respectively.) The method for manufacturing a non-oriented electrical steel sheet according to claim 9 , further comprising the step of heating the slab to 1200°C or less before the step of manufacturing the hot-rolled steel sheet. The method for producing a non-oriented electrical steel sheet according to claim 9 or 10 , wherein the finish rolling temperature in the step of producing the hot-rolled sheet is 800°C or higher. The method for manufacturing a non-oriented electrical steel sheet according to any one of claims 9 to 11 , further comprising the step of annealing the hot-rolled sheet at 850 to 1150°C after the step of manufacturing the hot-rolled sheet. The method for manufacturing a non-oriented electrical steel sheet according to any one of claims 9 to 12 , wherein the final annealing step comprises annealing at a soaking temperature of 900°C or more for 15 seconds or more. The method for manufacturing a non-oriented electrical steel sheet according to any one of claims 9 to 13 , wherein the final annealing step comprises annealing in an atmosphere containing 40 vol% or less of hydrogen ( _H2 ) and 60 vol% or more of nitrogen, and having a dew point of 0 to -40°C.